The present invention relates generally to thermoplastic, ABS-containing compositions which exhibit antistatic properties. More specifically, the present invention relates to antistatic compositions which include graft ABS copolymers and an epihalohydrin rubber.
Graft ABS copolymers have gained acceptance for a wide range of applications, such as computer housings, refrigerator linings and toys, due to their high impact strength, tensile strength and hardness combined with thermoplastic properties. However, most thermoplastic ABS compositions exhibit relatively slow electrostatic charge dissipation rates which make them unacceptable for many applications, such as certain medical devices and military equipment, wherein rapid dissipation of static electrical charge is required.
One method of reducing electrostatic charge retention is to coat the article, after molding, with materials which leave an antistatic film. While this may provide adequate antistatic properties temporarily, surface films tend to be worn off during use of the article, so that the film must be periodically reapplied or the antistatic properties are lost. This method also has the disadvantage of adding another step, e.g. coating, to the article manufacturing process, thereby increasing costs.
Another method of reducing static charge retention is that of incorporating a material with antistatic properties into the composition. One type of internal antistatic additive, such as ethoxylated amine chemicals, absorbs water from the air, thereby lowering the surface resistance of the composition. Unfortunately, however, this absorption process is reversed in dry environments, causing the antistatic property to diminish or be lost completely when the air surrounding the article is dry. These materials also tend to bloom to the surface of the article, where they are removed by wear.
Other additives used to reduce static charge retention are homopolymers and copolymers of ethylene oxide, such as epichlorohydrin rubber. Such polymers may be used as antistatic ingredients in plastics like polyvinyl chloride and other similar materials. U.S. Pat. No. 3,425,981 discloses a composition which includes an ethylene oxide homopolymer or copolymer in addition to an ethylene polymer resin. Suitable ethylene oxide copolymers contain ethylene oxide as the predominant monomer together with monomers having a single epoxy group, such as propylene oxide, butylene oxide and styrene oxide.
Certain antistatic compounds have also been added to ABS compositions in order to improve the electrostatic charge dissipation properties of the material. U.S. Pat. No. 3,450,794 to Ebneth, et al. discloses an antistatic thermoplastic composition which includes a graft copolymer of styrene and acrylonitrile on a conjugated diolefin, such as polybutadiene, a thermoplastic copolymer of a styrene and an acrylonitrile, blended with 1 to 10% by weight of a polypropylene glycol. This composition is reported as having a charge dissipation half life as low as 280 sec. However, this dissipation rate is not sufficient for many applications.
In summary, known means of imparting antistatic properties to plastics may have the disadvantages of diminishing with wear, leach from the thermoplastic by water or other solvents, exhibiting inadequate charge dissipation rates or having dissipation rates which are dependent on the amount of moisture present in the surrounding atmosphere. However, due to the pervasive use of plastic materials, a great need exists for durable, highly conductive antistatic thermoplastic materials. This problem is particularly acute with ABS thermoplastics which are in great demand due to their favorable physical properties, yet are such efficient insulators that they may be used in some applications to support live electrical parts.